Final answer:
The correct answer is a. notwithstanding, as it is a 15-letter word that means 'in spite of'.
Step-by-step explanation:
The correct answer to this question is a. notwithstanding. Notwithstanding is a 15-letter word that means 'in spite of'. It is often used to introduce a clause that contrasts with or contradicts a previous statement. For example, you could say 'I went to the party notwithstanding the rainy weather'. This means that despite the rain, you still went to the party. Other options like nevertheless and although can also indicate a contrast or contradiction, but they are not specifically synonymous with 'in spite of' like notwithstanding is.